So this achievement description is kinda misleading for a few people. There have been reports of people getting it when they get 25, 27 or 30 cores. Most of the people I've seen get it at around 27 cause it's the 25 main weapons plus the 2 new dlc weapons. The easiest way to farm cores is in public matches cause you'll have more people to play with and you have permanent damage buff if you play in public matches. Your new Bureau rank will correlate to how much damage you do in Tortured path public games (note that it does not apply to private games). To see your base damage go into the tortured path public lobby or host your own private lobby specifically in the tortured path mode and go to the soldier tab. The soldier tab will tell you your base damage. The max damage rank is unlocked at rank 50 which gives you 200% base damage and it'll tell you how many bosses you killed which is a good indicator of how many cores you have. The loadout I used for farming cores was Freefire with MK. II, fully loaded and resourceful along with using the 9mm sap. The 9mm sap was one of the best weapons to use against the bosses when pack a punched and when using free fire. Using free fire with MK. II on objectives rounds also makes it easier especially against the Meistermeuchlers if you get them in an early objective round.